It is known to use layered aluminosilicates or magnesium silicates in liquid cleansing and detergent compositions to modify the rheology of the composition. The art discloses, however, that these silicate clays are used to thicken (increase the viscosity) of the liquid composition. Typically, increased viscosity is accomplished by swelling or expanding of the silicates when they are in contact with water to form a “house of cards” structure. It is also commonly accepted (see for example, Luckham et al., Adv. Colloid Interface Sci, 82 (1999), 43-92.) that this house of cards structure arises from various interactions between the edges and faces of the layered clays.
Typically, the silicates (e.g., aluminosilicas) have a net negative charge on the basal plane in aqueous solutions, which arises from an isomorphous substitution of certain atoms in their structure for other atoms of a different valence. Thus, for example, Si4+ may be replaced by Al3+ in the lattice leading to a net negative charge on the basal plane. Such clays are called “cationic” clays because the net negative charge on the basal plane is compensated by the presence of cations in the interstitial layer. It is the hydration and dissolution of these interstitial cations that leads to swelling of the clays and the subsequent thickening effect for which they are commonly known. FIG. 1 is a schematic drawing showing typical structure with net anionic charge in the basal layer and cationic charge in the interstitial layer.
By contrast, in the subject invention it is critical that the clay be a non-silicate clay. A typical example of such clays are layered double hydroxides. Typically, these compounds have formula corresponding to the general formula:MgxAl(OH)yAznH2Oin which A represents an equivalent of a non-silicate anionic and the conditions 1<X<5, (y+z)=2x+3, 0<n<10 apply.
In such clays, the metal cation in the lattice is often substituted for one of a higher valence, which leads to a positive charge on the basal plane. This is thus the opposite of the silicates described above. Such clays are often referred to as “anionic clays” because the positive charge on the basal plane is balanced by the presence of anions like CO32− or NO3− in the interstitial layer.
In the present invention, applicants have found that the addition of the anionic (non-silicate) clay to personal wash compositions, unexpectedly, lowers the viscosity of the composition, thus acting as a hydrotrope (e.g., viscosity thinner rather than viscosity thickener). While not wishing to be bound by theory, the anionic clay is believed to help break down the mesostructure of surfactants used in the subject invention, thereby leading to easy dispersibility and ready incorporation of air (hence better foam).
U.S. Pat. No. 5,145,599 to Endres et al. does disclose the type of non-silicate layer compounds (having positive charge on basal layer) used in the subject invention. However, in the patent, the compounds are used in enzyme-containing, fabric-cleansing detergent compositions. Such detergent compositions are not concerned with lather, mildness or other attributes which the personal cleansing compositions of the subject invention relate to.
U.S. Pat. No. 5,661,189 to Grieveson et al. discloses that hydrotalcites can be used as a thickening agent (column 2, line 67). They are one of many potential “thickening agents” and there is clearly no teaching or suggestion that, if used at all, they serve to reduce viscosity. The present invention is a selection patent in which hydrotalcite must be used.